This invention relates to nuclear magnetic resonance (NMR) imaging methods. More specifically, this invention relates to method for controlling image artifacts due to substantially periodic NMR signal variations due, for example, to subject motion in the course of an NMR scan.
In the recent past, NMR has been developed as an imaging modality utilizing to obtain images of anatomical features of human patients, for example. Such images depicting nuclear spin distribution (typically, protons associated with water and tissue), spin-lattice relaxation time T.sub.1, and/or spin-spin relaxation time T.sub.2 are believed to be of medical diagnostic value in determining the state of health of the tissue examined. Imaging data for constructing NMR images can be collected using one of many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the NMR signal. As is well known, the NMR signal may be a free induction decay (FID) or, preferably, a spin-echo signal.
The preferred embodiments of the invention will be described in detail with reference to a variant of the FT technique, which is frequently referred to as "spin warp." It will be recognized, however, that the method of the invention is not limited to FT imaging methods, but may be advantageously practiced in conjunction with other techniques, such as multiple angle projection reconstruction disclosed in U.S. Pat. No. 4,471,306, and another variant of the FT technique disclosed in U.S. Pat. No. 4,070,611. The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein, et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). Briefly, the spin-warp technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), spatial information is encoded in one direction by applying a phase-encoding gradient along that direction and then observing a spin-echo signal in the presence of a magnetic field gradient in a direction orthogonal to the phase-encoding direction. The gradient present during the spin echo encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase-encoding gradient pulse is incremented monotonically in the temporal sequence of views.
Although it has been known that some NMR imaging pulse sequences produce artifacts due to object motion, early in the development of NMR imaging it was believed that among the advantages of the FT imaging method was its property of not producing motion artifacts. However, it is now well recognized that this is not so. Object motion during the acquisition of an NMR image produces both blurring and "ghosts" in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, including cardiac and respiratory motion, each NMR spin echo or FID can be considered a snapshot view of the object. Blurring and ghosts are due to the inconsistent appearance of the object from view to view.
Both deleterious effects of periodic motion, blurring and ghosts, can be reduced if the data acquisition is synchronized with the periodic motion. This method is known as gated scanning. Gating can also be used to study the mechanical dynamics of the motion itself, if that is of interest. The drawback of gating is that, depending on the period of the motion the fraction of the period during which acceptable data can be acquired, and the shortest acceptable pulse sequence repetition time, gating can significantly lengthen the data acquisition time.
While gating is required when the blurring due to the motion is unacceptable and when the motion itself is of interest (e.g., cardiac motion or flow), there are other applications where the loss of detail of the moving structures can be tolerated, but the disturbing effects of the ghosts which can extend far from the moving object cannot be accepted. In such applications, a method that can reduce or eliminate ghosts without the restrictions of gating is needed.
Ghost artifacts similar in character to those due to motion of the object portion being imaged can be caused by other substantially periodic variation in the NMR signals. Variations in the amplitude or phase of the received signals may be caused by changes in the RF coil characteristics due to motion of objects not under examination. Signal variations may also be caused by noise sources, e.g., line frequency noise whose phase varies from view to view in a substantially periodic manner. Reduction of these artifacts is also of interest and is within the scope of the present invention. Collectively, signal variations due to motion of the object being imaged as well as due to the indirect causes described above will be referred to hereinafter as signal variations.
One proposed method for eliminating ghost artifacts is disclosed and claimed in U.S. patent application Ser. No. 673,690, filed Nov. 21, 1984, now U.S. Pat. No. 4,567,893 and which is assigned to the same assignee as the present invention. In this case, it is recognized that the distance between the ghosts and the object imaged is maximized when the pulse sequence repetition time is an odd multiple one-fourth of the motion period (if two phase-alternated RF excitation pulses per view are used, as disclosed and claimed in commonly assigned U.S. Pat. No. 4,443,760, issued April 17, 1984). In the above-identified patent application, it is recognized that this ratio can be used to alleviate ghosts due to respiratory motion. While this method, indeed, improves image quality, it does impose a constraint on the pulse sequence repetition time used and usually results in a longer scan time.
With the projection reconstruction imaging techniques, substantially periodic motion again causes local distortion and blurring, as well as artifacts that extend well away from the moving structure. In this technique, the artifacts are manifested as streaks rather than ghosts. Once again, a method of reducing the distant effects can be of significant benefit.
Another method for reducing the undesirable effects due to periodic signal variations is disclosed and claimed in commonly assigned/concurrently filed U.S. patent application Ser. No. 766,842. In this method, a view increment time is selected, an assumption is made about the signal variation period (e.g., due, for example, to patient respiration) and a view order is generated prior to the beginning of the scan. View order selection involves establishing the order in which either the variable amplitude phase-encoding gradient pulses (in the spin-warp method) or the direction of the imaging gradient pulses (in multiple angle projection reconstruction method) are implemented. For a given signal variation period, and known view increment time, a view order is chosen so as to make the motion as a function of the phase-encoding amplitude or gradient direction be at any frequency. In one embodiment, the view order is selected such that the variation period appears to be equal to the scan time (low frequency) so that the ghost artifacts are brought as close to the moving object as possible. In another embodiment (high frequency), the view order is chosen to make the variation period appear to be equal to twice the pulse repetition time so as to attempt to push the ghost artifacts as far from the object as possible. Although this method is effective in reducing artifacts, and is in some respects ideal if the variation is rather regular and at a known frequency, the method is not very robust if the assumption made about the motion temporal period does not hold (e.g., because the patient's breathing pattern changes or is irregular). If this occurs, the method loses some of its effectiveness because the focussing of the ghosts, either as close to the object or as far from the object as possible, becomes blurred.
Accordingly, it is a principal object of the present invention to provide a method for achieving ghost artifact reduction or elimination which avoids the disadvantages of the aforedescribed methods.
It is another object of the present invention to provide a method for achieving ghost artifact reduction or elimination when the signal variations are not substantially periodic and/or the period of variation is not known a-priori.
It is yet another object of the present invention to provide a method for achieving ghost artifact reduction or elimination in which the view order is selected in real time based on measurements made during the scan.
It is still another object of the present invention to provide a method which is effective in achieving ghost artifact reduction or elimination while allowing complete freedom on the choice of pulse sequence repetition time.